1. Field of the Invention
The invention relates generally to electrical switching apparatus and, more particularly, to pole shaft assemblies for electrical switching apparatus, such as circuit breakers.
2. Background Information
Electrical switching apparatus, such as circuit breakers, provide protection for electrical systems from electrical fault conditions such as, for example, current overloads, short circuits, abnormal voltage and other fault conditions. Typically, circuit breakers include an operating mechanism which opens electrical contact assemblies to interrupt the flow of current through the conductors of an electrical system in response to such fault conditions as detected, for example, by a trip unit.
Some low-voltage circuit breakers, for example, employ a molded housing having two parts, a first half or front part (e.g., a molded cover), and a second half or rear part (e.g., a molded base). The operating mechanism for such circuit breakers is often mounted to the front part of the housing, and typically includes an operating handle and/or button(s) which, at one end, is (are) accessible from the exterior of the molded housing and, at the other end, is (are) coupled to a pivotable pole shaft. Electrical contact assemblies, which are also disposed within the molded housing, generally comprise a conductor assembly including a movable contact assembly having a plurality of movable contacts, and a stationary contact assembly having a plurality of corresponding stationary contacts.
When the movable contacts are rapidly separated from the stationary contacts, for example, in response to an overload or short circuit condition, an arc is created which generates gas that is at least partially ionized, as well as debris such as, for example, plasma material, molten and/or vaporized metal, and/or combustion products, such as carbon. Openings or gaps, for example, between the pole shaft and the circuit breaker housing allow the gas and debris, which are electrically conductive, to escape. If enough of the electrically conductive gas and/or debris is/are discharged near an electrically conductive structure, unintended arcing can occur. Among other undesirable effects associated with such arcing, it can present an unsafe condition for any individual attempting to operate the circuit breaker. It can also cause damage to circuit breaker electronics, melt plastic parts, and it can undesirably weld moving parts of the operating mechanism together, preventing the operating mechanism from operating properly. The debris can also become lodged, for example, in grease joints, causing friction which further impedes the operation of the operating mechanism and can cause wear and tear that reduces the mechanical life of the operating mechanism components. Furthermore, the escaping gas tends to form a shock wave, the pressure of which can cause damage to components of the circuit breaker. Failure to control the gap between the pole shaft and the housing also makes it difficult to control pressure and gas flow in the arc chamber, which is important to circuit breaker performance.
There is, therefore, room for improvement in electrical switching apparatus, such as circuit breakers, and in pole shaft assemblies therefor.